1. Field of the Invention
This invention is related to an apparatus for preventing the transmission of sound in a medium, and in particular a sound attenuator capable of attenuating low frequency sound in a hydrostatic pressure environment.
2. The Prior Art
A basic method of preventing the transmission of sound in a medium requires the introduction of a significant density discontinuity in the medium. For example, sound attenuation in a low density medium requires the introduction of a high density material, such as a slab of steel, to create a high density discontinuity in the low density medium. Similarly, in a high density medium, sound attenuation can be achieved by introducing a low density material, such as air, to create a low density discontinuity in the high density medium. Thus, a water/air interface would serve as an effective sound attenuator in water.
A captive arrangement of air bubbles can serve as a sound attenuator in a water medium. To be effective, the hydrostatic water pressure of a medium must not exceed the pressure required to collapse the bubbles. A layer of rubber containing air cavities has been used successfully as a sound attenuator in a water medium at hydrostatic pressures less than approximately 150 pounds per square inch (psi). Such sound attenuators are commonly referred to as air-rubber baffles.
An enclosure stiffer than rubber is required to attenuate sound in a water environment at pressures higher than 150 psi. At such pressures, the air bubbles would collapse and, thus, no sound would be attenuated. Enclosures stiff enough to withstand high levels of hydrostatic pressure should at the same time offer little or no resistance to sound pressure. Thus, if a stiff enclosure is constructed to exhibit resonant vibrations (with accompanying changes of the enclosed volume) at prescribed frequencies, the enclosure in effect becomes "soft" in the presence of sound pressure fluctuations at these prescribed frequencies. In other words, the enclosure is statically "stiff" but dynamically "soft." The vibration of the stiff enclosure absorbs the sound energy at the resonant frequency, but does not transmit this energy through the low density space inside the enclosure. An enclosure so designed thus acts as an efficient barrier against sound propagation at the prescribed resonant frequencies and therefore is considered a "tuned resonant baffle," commonly referred to as a "compliant tube baffle."
A conventional compliant tube is essentially a tube of near oval cross section whose long sides vibrate as plates and whose curved edges function as built-in nodes of vibration. Examples of conventional compliant tubes are shown in U.S. Pat. Nos. 3,264,605 and 3,907,062, the disclosures of which are hereby incorporated herein by reference. Such conventional compliant tubes are limited in their ability to attentuate relatively low frequency sound in a hydrostatic pressure environment. As explained in U.S. Pat. No. 3,264,605, as increasing pressure is exerted on the tube, the long walls of the tube bend toward the middle, and the curved or convex edges are drawn into a smaller arc. Thus, the pressure exerted on the tube forces the curved or convex edges into a smaller arc. The stress on the curved or convex edges increases with increasing pressure. Eventually, the pressure increases to a value that causes the tube to rupture and renders the tube useless.
The maximum static pressure which the tube is capable of withstanding can be increased by making the curved edges thicker. However, increasing the thickness of the tube wall at the edges results in two disadvantages: increased weight and an increase in the tuned frequency of the tube. Extra weight is undesirable in submarine applications such as the application disclosed in U.S. Pat. No. 3,907,062. The greater thickness of the tube wall also precludes the attenuation of low frequencies (i.e., less than 1000 Hz) because the frequencies below which attentuation does not occur increase with increasing wall thickness. Thus, thickening the tube walls is not an acceptable solution in applications requiring attenuation of lower frequencies in hydrostatic pressure environments.
The solution of U.S. Pat. No. 3,264,605 of introducing a relatively noncompressiole fluid inside the tube is marginally effective to prevent rupturing of the tube at higher pressures than the tubes could withstand without the presence of the noncompressible fluid. However, in some applications, the added weight of the noncompressible fluid is as equally undesirable as is the weight added by the increased thickness of the tube wall. Moreover, the frequencies capable of being attenuated by the tube containing a noncompressible fluid are relatively higher than frequencies capable of being attenuated by a hollow tube which is similar in all respects except for the presence of the noncompressible fluid.